Recently, as having the effects of peeling off an old stratum corneum in a skin surface, regenerating a new stratum corneum, and thus enhancing turnover (metabolism) of the epidermis, α-hydroxy acids, such as glycolic acid, lactic acid, malic acid, and tartaric acid, salicylic acid, trichloroacetic acid, phenol, and the like, have been contained in external preparations for skin, typically cosmetics. Also, the above-mentioned compounds are used for improvement of skin texture, a somber color on the skin, skin roughness, or the like, and for chemical peeling for the purposes of improvement of fine wrinkles, various types of pigmentation, such as spots, flecks, chloasmata, and senile pigment freckles, acne, dermatitis traces, fire burns, heat burns, traumata, and removal of wrinkles or spots on scar skins thereof. (Reference may be made to, for example, Patent Documents 1 and 2.) In order for the skin texture, the somber color on the skin, the acne, and the like, to be improved by use of the above-mentioned drugs, it is preferable that the drugs remain on the skin. However, the conventional external preparations for the chemical peeling have the problems in that the external preparations drip at the time of their use, and therefore their usability is bad. Also, in cases where large quantities of thickeners are contained in the external preparations for the thickening thereof, the problems are encountered in that the external preparations are difficult to wash away at the time of removal thereof.
As for techniques with which the thickeners are contained in the external preparations containing the α-hydroxy acids, and the like, there have heretofore been known, for example, attempts for increasing viscosities in compositions containing the α-hydroxy acids by the containing of a carboxyvinyl polymer and an α-hydroxy acid in a specific containing ratio, or by the containing of an α-hydroxy carboxylic acid, a heterobio-polysaccharide gum, such as a xanthan gum, an inorganic thickener, and a polyacrylamide in specific containing quantities. (Reference may be made to Patent Documents 3 and 4.) Further, in α-hydroxy acid containing external preparations which are currently available commercially, a xanthan gum or hydroxyethyl cellulose is used as the thickener.
However, the aforesaid conventional external preparations have bad viscosity stability and have the problems in that, though the external preparations have a high viscosity at the stage immediately after the production, the viscosity decreases markedly during storage, particularly during storage at high temperatures. Particularly, in cases the conventional external preparations contain the compounds, such as the α-hydroxy acids, in large containing quantities and have low pH values, it is not always possible for the external preparations to accomplish sufficient and stable thickening. Also, in cases where the quantities of the thickeners are increased, and the viscosities at the time of the production are set to be excessively high, the problems occur in that the usability of the external preparations becomes bad, or in that the external preparations are difficult to wash away at the time of removal from the skin.